Källén and Not Richard Feynman But His Diagrams

Kallen didn’t approve of Schwinger because of what he considered to be his “unethical behavior” in their planned collaboration. I (CJ) heard him say so! What about Feynman? In his correspondence (the Kallen Collection) one finds no answer to this question and there is no evidence that he ever criticized Feynman. However, he would sometimes warn the students about inadvertent use of Feynman diagrams. Field theory for him was more that just a collection of such diagrams. The diagrams were useful, as he noted in his article about the 1965 Nobel Prize in Physics and in the last few years of his life he used them, for example, in his lectures at 1966 Schladming School. His point was that you can get, for example, absurd values for the photon mass by basing your computation on the corresponding Feynman diagram. In the Kallen Collection a number of letters and reports deal with this topic. As an example, there is a copy of a letter from Herbert M. Fried, dated 24 Nov. 1959, and addressed to the editor of Physical Review, S. Goudsmit, in which he writes: “ … Finally, I would like to thank the referee for pointing out two glaring errors in the original manuscript. … The final remark of the Referee concerning the operator gauge transformation involving the constant L is also gratefully acknowledged.”